95 
PHOTOGRAPHIC INTENSITY OF SUNSHINE. 
A photographic method of determining the brightness of sunshine 
or sky light is very desirable as supplementing the thermometric 
methods. It is as erroneous to assume that all radiation that falls 
upon a black-bulb thermometer is absorbed by it and converted into 
heat and measured by the expansion of the mercury as it is to assume 
that all the radiation that falls on a photographic film is absorbed 
by it and is represented by the chemical changes that take place in the 
film. Equally erroneous would it be to assume that all the radiation 
that enters the eye is represented by the impression of brightness 
conveyed by the retina to the brain. In order to measure in absolute 
units the total energy radiated from the sun, we need a proper 
summation of the thermal, visual, and photographic work done by 
the radiation. If we wish to determine only the intensity of that 
part of the radiation that does the work in which agriculture 1s 
chiefly interested we should consider only the heating effects of the 
radiation and the special chemical effects manifested in the action of 
sunlight upon chlorophyll. 
The action of the sunlight upon the chlorides and bromides of 
silver, as in ordinary photographic processes, may not be an exact 
measure of its action upon the leaves of plants. Some other chemicals 
may be more appropriate for use at agricultural experiment stations, 
but the photographic methods perfected by Profs. H. W. Vogel and 
L. Weber are worthy of trial as a first step in the right direction. 
These processes give us the relative intensity ‘of the radiations that 
belong to the blue end of the spectrum, with only a small admixture 
of the influence of green and yellow rays. | 
During the year 1890, as the result of a numerous series of observa- 
tions at Kiel, Prof. L. Weber found that the reddish light of the 
spectrum on dark winter days has only about 500 times greater inten- 
sity than the quantity of light from a normal candle at a distance of 
1 meter, when measured by their relative effects on a photographic 
plate, while at the same time the photographic intensity of the green 
light of the spectrum was four times as much. On bright summer 
days the intensity of the red ight was 50,000 times that of the candle 
at 1 meter, while the intensity of the green light was about 200,000, 
or about 4 times as much in summer as in winter. The intensity of 
the blue hight in the solar spectrum was about 25 times that of the 
red light, which ratio varied a little with the kind and amount of 
cloud. In all this photographic work a very sensitive silver bromide 
paper was used; so that these results, strictly speaking, relate only to 
the variations in the intensity of those special rays that affect this 
chemical. But these variations will be nearly parallel to the diurnal 
and annual variations of the rays that affect the growth of plants. 
